An opposition with a third body squaring both ends. Three of the four points of a grand cross, with the fourth left empty.
Two bodies 180° apart is an opposition on its own. A third body 90° from each of them turns it into a T: the two opposed bodies are the crossbar, the third is the upright, sitting at what the tradition calls the apex — the point catching pressure from both directions rather than radiating it.
Because 90° and 180° are both multiples of 90°, a T-square's three bodies most often sit in three signs of the same modality — the fourth sign of that modality, directly opposite the apex, is the 'missing leg' where a fourth body would complete a grand cross. Whether that opposite sign is empty or occupied by something outside this site's pattern orb is worth checking directly on the wheel.
The apex is the body this site names in the summary line, and it's the traditional focus of the figure: the one place in the chart absorbing two 90° angles rather than contributing to the more comfortable opposition between the other two.
Traditionally attributed
Sustained pressure with a single outlet — traditionally read as friction that, unlike an isolated square, does not have a second point of relief to dissipate through.
Listed because it is what the tradition says, not because this site can check it. Everything above this line is geometry; everything in it is convention.